1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus.
2. Description of the Related Art
FIG. 1 in Japanese Patent Application Laid-Open No. 2000-77644 illustrates a photoelectric conversion apparatus using a phototransistor. In the Figure, a constant current source and a MOSFET driven by the constant current source provide a common source circuit, and the gate-source voltage of the MOSFET determines the base potential of the phototransistor. In the Figure, when the amount of light changes, the emitter current of the phototransistor changes, and thus, the base-emitter voltage changes. In such case, not the base potential but the emitter potential of the phototransistor mainly changes. Accordingly, the base potential is maintained substantially constant during operation once the base potential is set to a stationary voltage.
However, in the Figure, when power is applied, that is, when a power supply terminal voltage is set to a source voltage from 0 V, a long time is required for the base potential to reach a stationary voltage from 0 V. This is because time is consumed for charging a parasitic capacitance parasitic in the base with a photocurrent of the phototransistor: as the luminance becomes lower, more time is required because the photocurrent is smaller. Consequently, a very long time is required for the emitter current of the phototransistor (an output current of a pixel) to converge. Therefore, normally, a unit that resets the base potential is provided as in the Figure (for example, an MOSFET with an a source-drain terminal connected to the base and the other terminal connected to a reset potential), and by turning on the MOSFET when power is applied, the base potential is reset so as to have a value slightly higher than the stationary voltage. In such case, because of the characteristics of a common source circuit, the gate potential of the MOSFET is lowered, whereby the emitter potential of the phototransistor being lowered. Consequently, a large current is output from the emitter of the phototransistor. After cancellation of the reset, that is, after turning off the MOSFET, the base current is larger than the photocurrent because of the large emitter current, and thus, excess charge is drawn out from the base, and the base potential is gradually stabilized to the stationary voltage. Furthermore, concurrently with that, the emitter current Ie also converges (decreases), and finally converges on a value obtained by expression (1) below.[Expression 1]Ie≈hFE×(Ip+Id)  (1)
Here, hFE is a current amplification factor of the phototransistor, Ip is the photocurrent, and Id is a dark current. The temporal waveform of the emitter current Ie shows a waveform in which an excessive current is generated in a reset period and the excessive current is gradually lowered and finally converges on a value determined by expression (1). From expression (1), the converged value of the emitter current Ie depends on the photocurrent Ip, and as the photocurrent Ip is lower, time is required more for convergence. Even when such reset operation is performed, a long time is still required for convergence of the emitter current Ie.